The present invention relates to very high areal density magnetic recording media exhibiting improved thermal stability, such as hard disks. More particularly, the present invention relates to longitudinal, anti-ferromagnetically coupled (xe2x80x9cAFCxe2x80x9d) magnetic recording media including improved spacer layers providing reduced or optimized lattice mismatch with vertically spaced-apart ferromagnetic layers.
Magnetic recording (xe2x80x9cMRxe2x80x9d) media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form. Conventional thin-film type magnetic media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer, are generally classified as xe2x80x9clongitudinalxe2x80x9d or xe2x80x9cperpendicularxe2x80x9d, depending upon the orientation of the magnetic domains of the grains of magnetic material.
A portion of a conventional longitudinal recording, thin-film, hard disk-type magnetic recording medium 1 commonly employed in computer-related applications is schematically illustrated in FIG. 1 in simplified cross-sectional view, and comprises a substantially rigid, non-magnetic metal substrate 10, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Alxe2x80x94Mg) alloy, having sequentially deposited or otherwise formed on a surface 10A thereof a plating layer 11, such as of amorphous nickel-phosphorus (Nixe2x80x94P); a seed layer 12A of an amorphous or fine-grained material, e.g., a nickel-aluminum (Nixe2x80x94Al) or chromium-titanium (Crxe2x80x94Ti) alloy; a polycrystalline underlayer 12B, typically of Cr or a Cr-based alloy, a magnetic recording layer 13, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer 14, typically containing carbon (C), e.g., diamond-like carbon (xe2x80x9cDLCxe2x80x9d); and a lubricant topcoat layer 15, e.g., of a perfluoropolyether. Each of layers 11-14 may be deposited by suitable physical vapor deposition (xe2x80x9cPVDxe2x80x9d) techniques, such as sputtering, and layer 15 is typically deposited by dipping or spraying.
In operation of medium 1, the magnetic layer 13 is locally magnetized by a write transducer, or write xe2x80x9cheadxe2x80x9d, to record and thereby store data/information therein. The write transducer or head creates a highly concentrated magnetic field which alternates direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer 13, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium layer 13 can subsequently produce an electrical response in a read transducer, or read xe2x80x9cheadxe2x80x9d, allowing the stored information to be read.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (xe2x80x9cSMNRxe2x80x9d) of the magnetic media. However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in2 in order to form ultra-high recording density media, such as thermal instability, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits.
One proposed solution to the problem of thermal instability arising from the very small grain sizes associated with ultra-high recording density magnetic recording media, including that presented by the superparamagnetic limit, is to increase the crystalline anisotropy, thus the squareness of the magnetic bits, in order to compensate for the smaller grain sizes. However, this approach is limited by the field provided by the writing head.
Another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide stabilization via coupling of the ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer. In this regard, it has been recently proposed (E. N. Abarra et al., IEEE Conference on Magnetics, Toronto, April 2000) to provide a stabilized magnetic recording medium comprised of at least a pair of ferromagnetic layers which are anti-ferromagnetically-coupled (xe2x80x9cAFCxe2x80x9d) by means of an interposed thin, non-magnetic spacer layer. The coupling is presumed to increase the effective volume of each of the magnetic grains, thereby increasing their stability; the interface exchange energy density, J, between the ferromagnetic layer pairs being a key parameter in determining the increase in stability.
However, a drawback associated with the above-described approach is encountered when a number of materials, e.g., Ru, are utilized as the non-magnetic spacer layer for providing AFC between Co-based ferromagnetic layers, as are typically employed in the fabrication of high areal density magnetic recording media. Illustratively, the lattice constants of hexagonal close-packed (xe2x80x9chcpxe2x80x9d) Ru are a=2.714xc3x85 and c=4.299 xc3x85, which lattice constants are frequently much larger than the corresponding lattice constants of the typically employed hcp Co-based ferromagnetic layers. Thus, in order to obtain a desired crystallographic orientation (e.g., an in-plane alignment of the c-axis) and microstructure of the Co-based ferromagnetic layer(s) grown on the non-magnetic spacer layer, the mismatch between the lattice constants of the non-magnetic spacer layer and the Co-based ferromagnetic layers on opposite sides of the non-magnetic spacer layer must be adjusted, i.e., reduced or optimized, in order to obtain optimal or maximal performance of the AFC media.
Accordingly, there exists a need for improved methodology for providing thermally stable, high areal density magnetic recording media, e.g., longitudinal media, with large interface exchange energy density, J, optimal microstructure and crystallographic orientation (i.e., in-plane alignment of the c-axis), and reduced or optimized lattice mismatch between vertically separated ferromagnetic layers and a non-magnetic spacer layer (such as of a Ru-based material) providing anti-ferromagnetic coupling (AFC) of the ferromagnetic layers, wherein each of the ferromagnetic layers is formed of a ferromagnetic alloy composition similar to compositions conventionally employed in fabricating longitudinal magnetic recording media, which methodology can be implemented at a manufacturing cost compatible with that of conventional manufacturing technologies for forming high areal density magnetic recording media. There also exists a need for improved, high areal density magnetic recording media, e.g., in disk form, which media include at least one pair of anti-ferromagnetically coupled ferromagnetic alloy layers separated by a non-magnetic spacer layer, wherein each of the ferromagnetic layers is formed of a ferromagnetic alloy composition similar to compositions conventionally utilized in longitudinal magnetic recording media (such as Co-based alloys) and the lattice mismatch between each of the ferromagnetic layers and the non-magnetic spacer layer is reduced or optimized, leading to improved thermal stability.
The present invention, therefore, addresses and solves problems attendant upon forming high areal recording density magnetic recording media, e.g., in the form of hard disks, which media utilize anti-ferromagnetic coupling between vertically spaced-apart pairs of ferromagnetic layers for enhancing thermal stability, while providing full compatibility with all aspects of conventional automated manufacturing technology. Moreover, manufacture and implementation of the present invention can be obtained at a cost comparable to that of existing technology.
An advantage of the present invention is an improved, high areal recording density magnetic recording medium having increased thermal stability and signal-to-medium noise ratio (xe2x80x9cSMNRxe2x80x9d).
Another advantage of the present invention is an improved, high areal recording density, longitudinal magnetic recording medium having reduced or optimized lattice mismatch between a pair of vertically spaced-apart ferromagnetic layers and a non-magnetic spacer layer providing anti-ferromagnetic coupling between the spaced-apart ferromagnetic layers.
Yet another advantage of the present invention is an improved method for manufacturing a high areal recording density magnetic recording medium having increased thermal stability and signal-to-medium noise ratio (xe2x80x9cSMNRxe2x80x9d).
Still another advantage of the present invention is an improved method for manufacturing a high areal recording density, longitudinal magnetic recording medium having reduced or optimized lattice mismatch between a pair of vertically spaced-apart ferromagnetic layers and a non-magnetic spacer layer providing anti-ferromagnetic coupling between the spaced-apart ferromagnetic layers. Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized as particularly pointed out in the appended claims.
According to one aspect of the present invention, the foregoing and other advantages are obtained in part by an anti-ferromagnetically coupled (AFC) magnetic recording medium having improved thermal stability and signal-to-medium noise ratio (SMNR), comprising:
a non-magnetic substrate having at least one surface; and
a layer stack overlying the at least one surface, the layer stack comprising at least one layer pair comprised of first and second superposed, crystalline ferromagnetic layers spaced-apart by a crystalline, non-magnetic spacer layer providing anti-ferromagnetic coupling (AFC) between the first and second ferromagnetic layers;
wherein the lattice constants of the crystalline, non-magnetic spacer layer are substantially matched to the lattice constants of each of the first and second crystalline ferromagnetic layers for obtaining a desired crystalline orientation and microstructure of the ferromagnetic layers without significantly degrading the interface exchange energy density J of the AFC.
According to embodiments of the present invention, each of the first and second ferromagnetic layers comprises a single ferromagnetic layer or a plurality of ferromagnetic sub-layers, wherein each of the first and second ferromagnetic layers comprises at least one ferromagnetic alloy, e.g., at least one Co-based alloy, and the c-axis in-plane alignment of the at least one Co-based alloy is improved as a result of the lattice matching.
In accordance with the present invention, Co-based alloys include alloys of Co with at least one element selected from the group consisting of Pt, Cr, B, Fe, Ta, Ni, Mo, V, Nb, Ru, Si, and Ge; and the non-magnetic spacer layer comprises an alloy, e.g., one or more layers of one or more Ru-based alloys, such as alloys of Ru with at least one of Cr, Rh, Ir, and Ta.
In accordance with particular embodiments of the present invention, the single ferromagnetic layer and each of the plurality of ferromagnetic sub-layers comprises an alloy of Co with at least one element selected from the group consisting of Pt, Cr, B, Fe, Ta, Ni, Mo, V, Nb, Ru, Si, and Ge; and the non-magnetic spacer layer comprises one or more layers of one or more Ru-based alloys selected from Ru alloyed with at least one element selected from Cr, Rh, Ir, and Ta or a Ru-based alloy layer wherein the concentration of the alloying element(s) is continuously varied across the thickness of the layer; each of the first and second ferromagnetic layers is from about 8 to about 300 xc3x85 thick; and the non-magnetic spacer layer is from about 4 to about 20 xc3x85 thick.
According to further embodiments of the present invention, the layer stack includes a plurality of layer pairs, each comprised of the superposed first and second crystalline ferromagnetic layers spaced-apart and anti-ferromagnetically coupled together by a crystalline non-magnetic spacer layer having lattice constants which are substantially matched to the lattice constants of the first and second ferromagnetic layers.
In accordance with still further embodiments of the present invention, the magnetic recording medium further comprises seed and underlayers between the at least one surface of the substrate and the layer stack for controlling the crystallographic texture of the at least one layer pair of first and second crystalline ferromagnetic layers; and protective overcoat and lubricant topcoat layers provided on an upper surface of the layer stack.
According to another aspect of the present invention, a method of forming a magnetic recording medium having improved thermal stability and signal-to-media noise ratio (SMNR), comprises steps of:
(a) forming a layer stack overlying at least one surface of a non-magnetic substrate, the layer stack including at least one layer pair comprised of first and second superposed, spaced-apart, crystalline ferromagnetic layers; and
(b) providing a crystalline, non-magnetic spacer layer between the first and second ferromagnetic layers for inducing anti-ferromagnetic coupling (AFC) of the first and second ferromagnetic layers, the lattice constants of the crystalline, non-magnetic spacer layer being substantially matched to the lattice constants of each of the first and second crystalline ferromagnetic layers for obtaining a desired crystalline orientation and microstructure of the ferromagnetic layers without significantly degrading the interface exchange energy density J of the AFC.
According to embodiments of the present invention, step (a) comprises forming a layer stack wherein each of the first and second ferromagnetic layers is comprised of a single ferromagnetic layer or a plurality of ferromagnetic sub-layers, wherein each of the first and second ferromagnetic layers comprises at least one ferromagnetic alloy; and step (b) comprises providing a crystalline, non-magnetic alloy spacer layer.
In accordance with particular embodiments of the present invention, the at least one ferromagnetic alloy is a Co-based alloy and the c-axis in-plane alignment of the at least one Co-based alloy layer is improved as a result of the lattice matching; and the non-magnetic alloy spacer layer comprises one or more layers of one or more Ru-based alloys or a Ru-based alloy layer wherein the concentration of the alloying element(s) is continuously varied across the thickness of the layer. For example, the at least one Co-based alloy comprises Co alloyed with at least one element selected from the group consisting of Pt, Cr, B, Fe, Ta, Ni, Mo, V, Nb, Ru, Si, and Ge; and the Ru-based alloys are selected from Ru alloyed with at least one element selected from Cr, Rh, Ir, and Ta.
According to further particular embodiments of the present invention, step (a) comprises providing a layer stack wherein each of the first and second ferromagnetic layers is from about 8 to about 300 xc3x85 thick; and step (b) comprises providing a crystalline, non-magnetic spacer layer from about 4 to about 20 xc3x85 thick.
According to still further embodiments of the present invention, step (a) comprises providing the layer stack as including a plurality of the layer pairs, each comprised of superposed, spaced-apart, crystalline first and second ferromagnetic layers; and step (b) comprises providing a crystalline, non-magnetic spacer layer between each of the first and second ferromagnetic layers of each layer pair for inducing anti-ferromagnetic coupling therebetween.
In accordance with yet further embodiments of the present invention, step (a) further comprises providing seed and underlayers between the at least one surface of the substrate and the layer stack for controlling the crystallographic texture of the at least one layer pair of first and second crystalline ferromagnetic layers; and the method further comprises the step of: (c) providing protective overcoat and lubricant topcoat layers on an upper surface of the layer stack.
Still another aspect of the present invention is a high areal density magnetic recording medium having improved thermal stability and signal-to-medium noise ratio (SMNR), comprising:
at least one pair of superposed, spaced-apart, anti-ferromagnetically coupled (AFC) crystalline ferromagnetic layers; and
means for obtaining a desired crystalline orientation and microstructure of the ferromagnetic layers without significantly degrading the interface exchange energy density J of the AFC.